Conversion of low molecular weight alkanes, such as methane, to synthetic fuels or chemicals has received increasing attention as low molecular weight alkanes are generally available from secure and reliable sources. For example, natural gas wells and oil wells currently produce vast quantities of methane. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during mining operations, in petroleum processes, and in the gasification or liquefaction of coal, tar sands, oil shale, and biomass.
Many of these alkane sources are located in relatively remote areas, far from potential users. Accessibility is a major obstacle to effective and extensive use of remotely situated methane, ethane and natural gas. Costs associated with liquefying natural gas by compression or, alternatively, constructing and maintaining pipelines to transport natural gas to users are often prohibitive. Consequently, methods for converting low molecular weight alkanes to more easily transportable liquid fuels and chemical feedstocks are desired and a number of such methods have been reported.
Reported methods can be conveniently categorized as direct oxidation routes and/or as indirect syngas routes. Direct oxidative routes convert lower alkanes to products such as methanol, gasoline, and relatively higher molecular weight alkanes. In contrast, indirect syngas routes involve, typically, production of synthesis gas as an intermediate product.
As is well known in the art, synthesis gas ("syngas") is a mixture of carbon monoxide and molecular hydrogen, generally having a dihydrogen to carbon monoxide molar ratio in the range of 1:5 to 5:1, and which may contain other gases such as carbon dioxide. Synthesis gas has utility as a feedstock for conversion to alcohols, olefins, or saturated hydrocarbons (paraffins) according to the well known Fischer-Tropsch process, and by other means. Synthesis gas is not a commodity; rather, it is typically generated on-site for further processing. At a few sites, synthesis gas is generated by a supplier and sold "over the fence" for further processing to value added products. One potential use for synthesis gas is as a feedstock for conversion to high molecular weight (e.g. C.sub.50+) paraffins which provide an ideal feedstock for hydrocracking for conversion to high quality jet fuel and superior high cetane value diesel fuel blending components. Another potential application of synthesis gas is for large scale conversion to methanol.
In order to produce high molecular weight paraffins in preference to lower molecular weight (e.g. C.sub.8 to C.sub.12) linear paraffins, or to synthesize methanol it is desirable to utilize a synthesis gas feedstock having an H.sub.2 :CO molar ratio of about 2.1:1, 1.9:1, or less. As is well known in the art, Fischer-Tropsch syngas conversion reactions using syngas having relatively high H.sub.2 :CO ratios produce hydrocarbon products with relatively large amounts of methane and relatively low carbon numbers. For example, with an H.sub.2 :CO ratio of about 3, relatively large amounts of C1-C8 linear paraffins are typically produced. These materials arc characterized by very low octane value and high Reid vapor pressure, and are highly undesirable for use as gasoline.
Lowering the H.sub.2 :CO molar ratio alters product selectivity by increasing the average number of carbon atoms per molecule of product, and decreasing the amount of methane and light paraffins produced. Thus, it is desirable for a number of reasons to generate syngas feedstocks having molar ratios of hydrogen to carbon monoxide of about 2:1 or less.
Prior methods for producing synthesis gas from natural gas (typically referred to as "natural gas reforming") can be categorized as (a) those relying on steam reforming where natural gas is reacted at high temperature with steam, (b) those relying on partial oxidation in which methane is partially oxidized with pure oxygen by catalytic or non-catalytic means, and (c) combined cycle reforming consisting of both steam reforming and partial oxidation steps.
Steam reforming involves the high temperature reaction of methane and steam over a catalyst to produce carbon monoxide and hydrogen. This process, however, results in production of syngas having a high ratio of hydrogen to carbon monoxide, usually in excess of 3:1.
Partial oxidation of methane with pure oxygen provides a product which has an H.sub.2 :CO ratio close to 2:1, but large amounts of carbon dioxide and carbon are co-produced, and pure oxygen is an expensive oxidant. An expensive air separation step is required in combined cycle reforming systems, although such processes do result in some capital savings since the size of the steam reforming reactor is reduced in comparison to a straightforward steam reforming process.
Although direct partial oxidation of methane using air as a source of oxygen is a potential alternative to today's commercial steam-reforming processes, downstream processing requirements cannot tolerate nitrogen (recycling with cryogenic separations is required), and pure oxygen must be used. The most significant cost associated with partial oxidation is that of the oxygen plant. Any new process that could use air as the feed oxidant and thus avoid the problems of recycling and cryogenic separation of nitrogen from the product stream will have a dominant economical impact on the cost of a syngas plant, which will be reflected in savings of capital and separation costs.
Thus, it is desirable to lower the cost of syngas production as by, for example, reducing the cost of the oxygen plant, including eliminating the cryogenic air separation plant, while improving the yield as by minimizing the co-production of carbon, carbon dioxide and water, in order to best utilize the product for a variety of downstream applications.
Dense ceramic membranes represent a class of materials that offer potential solutions to the above-mentioned problems associated with natural gas conversion. Certain ceramic materials exhibit both electronic and ionic conductivities (of particular interest is oxygen ion conductivity). These materials not only transport oxygen (functioning as selective oxygen separators), but also transport electrons back from the catalytic side of the reactor to the oxygen-reduction interface. As such, no external electrodes are required, and if the driving potential of transport is sufficient, the partial oxidation reactions should be spontaneous. Such a system will operate without the need of an externally applied electrical potential. Although there are recent reports of various ceramic materials that could be used as partial oxidation ceramic membrane, little work appears to have been focused on the problems associated with the stability of the material under methane conversion reaction conditions.
European Patent Application 90305684.4, published on Nov. 28, 1990, under Publication No. EP 0 399 833 A1 in the name of Cable et al., describes an electrochemical reactor using solid membranes comprising: (1) a multi-phase mixture of an electronically-conductive material, (2) an oxygen ion-conductive material, and/or (3) a mixed metal oxide of a perovskite structure. Reactors are described in which oxygen from oxygen-containing gas is transported through a membrane disk to any gas that consumes oxygen. Flow of gases on each side of the membrane disk in the reactor shell shown are symmetrical flows across the disk, substantially radial outward from the center of the disk toward the wall of a cylindrical reactor shell. The gases on each side of the disk flow parallel to, and co-current with, each other.
Materials known as "perovskites" are a class of materials which have an X-ray identifiable crystalline structure based upon the structure of the mineral perovskite, CaTiO.sub.3. In its idealized form, the perovskite structure has a cubic lattice in which a unit cell contains metal ions at the corners of the cell, another metal ion in its center and oxygen ions at the midpoints of each cube edge. This cubic lattice is identified as an ABO.sub.3 -type structure where A and B represent metal ions. In the idealized form of perovskite structures, generally, it is required that the sum of the valences of A ions and B ions equal 6, as in the model perovskite mineral, CaTiO.sub.3.
A variety of substitutions of the A and B cations can occur. Replacing part of a divalent cation by a trivalent cation or a pentavalent ion for a tetravalent ion, i.e., donor dopant, results in two types of charge compensation, namely, electronic and ionic, depending on the partial pressure of oxygen in equilibrium with the oxides. The charge compensation in acceptor-doped oxides, i.e., substituting a divalent cation for a trivalent cation is, by electronic holes, at high oxygen pressures, but at low pressures, it is by oxygen ion vacancies. Ion vacancies are the pathway for oxide ions. Therefore, the oxygen flux can be increased by increasing the amount of substitution of lower valence element for a higher valence metal ion. The reported oxygen flux values in perovskites tend to follow the trends suggested by the charge compensation theory. While the primary property of high oxygen flux appears to be feasible in a few combination of dopants in AB03 type oxides, many other questions need to be answered about the ideal material for constructing a novel membrane reactor. For example, the mechanical properties of the chosen membrane must have the strength to maintain integrity at the conditions of reaction. It must also maintain chemical stability for long periods of time at the reaction conditions. The oxygen flux, chemical stability, and mechanical properties depend on the stoichiometry of the ceramic membrane.
Many materials having the perovskite-type structure (ABO.sub.3 -type) have been described in recent publications including a wide variety of multiple cation substitutions on both the A and B sites as being stable in the perovskite structure. Likewise, a variety of more complex perovskite compounds containing a mixture of A metal ions and B metal ions (in addition to oxygen) are reported. Publications relating to perovskites include: P. D. Battle et al., J. Solid State Chem., 76, 334 (1988); Y. Takeda et al., Z. Anorg. Allg. Chem., 550/541, 259 (1986); Y. Teraoka et al., Chem. Lett., 19, 1743 (1985); M. Harder and H. H. Muller-Buschbaum, Z. Anorg. Allg. Chem., 464, 169 (1980); C. Greaves et al., Acta Cryst., B31, 641 (1975).
For example, Hayakawa et al. U.S. Pat. No. 5,126,499, incorporated by reference herein, describes a process for the production of hydrocarbons by oxidative coupling of methane using a perovskite-type oxide having the following composition: EQU M.sub.1 (Co.sub.1-x Fe.sub.x).sub.1 O.sub.y
where M stands for at least one alkaline earth metal, x is a number greater than 0, but not greater than 1, and y is a number in the range of 2.5-3.5 at a temperature of 500.degree. to 1000.degree. C.
Commonly assigned U.S. Pat. Nos. 5,580,497 and 5,639,437 in the names of Uthamalingam Balachandran, Mark S. Kleefisch, Thaddeus P. Kobylinski, Sherry L. Morissette and Shiyou Pei, incorporated by reference herein, discloses preparation, structure and properties of a class of mixed metal oxide compositions of at least strontium, cobalt, iron and oxygen, and is incorporated herein by reference in its entirety. Use of the mixed metal oxides in dense ceramic membranes having electron conductivity and oxygen ion conductivity, are described as well as their use in separation of oxygen from an oxygen-containing gaseous mixture to form an oxygen-depleted first product and optionally reacting recovered oxygen with organic compounds in another gaseous mixture.
Ceramic powders with varying stoichiometry are made by solid-state reaction of the constituent carbonates and nitrates. Appropriate amounts of reactants are, generally, mixed and milled in methanol using zirconia media for several hours. After drying, the mixtures are calcined in air at elevated temperatures, e.g., up to about 850.degree. C. for several hours, typically, with an intermittent grinding. After the final calcination, the powder is ground to small particle size. The morphology and particle size distribution can play a significant role during the fabrication of membrane tubes.
Membrane tubes can be conveniently fabricated by known methods of plastic extrusion. To prepare for extrusion, ceramic powder is, generally, mixed with several organic additives to make a formulation with enough plasticity to be easily formed into various shapes while retaining satisfactory strength in the green state. This formulation, known as a slip, consists in general of a solvent, a dispersant, a binder, a plasticizer, and ceramic powder. The role of each additive is described in Balachandran et al., Proceedings International Gas Research Conference, Orlando, Fla. (H. A. Thompson editor, Government Institutes, Rockville, Md.), pp. 565-573 (1992). Ratios of the various constituents of a slip vary, depending on the forming process and such characteristics of the ceramic powder as particle size and specific surface area. After the slip is prepared, some of the solvent is allowed to evaporate; this yields a plastic mass that is forced through a die at high pressure (about 20 MPa) to produce hollow tubes. Tubes have been extruded with outside diameters of about -6.5 mm and lengths up to about 30 cm. The wall thicknesses are in the range 0.25 to 1.20 mm. In the green state (i.e., before firing), extruded tubes exhibit great flexibility.
Extruded tubes are heated at a slow heating rate (5.degree. C./h) in the temperature range 150.degree. to 400.degree. C. to facilitate removal of gaseous species formed during decomposition of organic additives. After the organics are removed at low temperatures, the heating rate is increased to about 60.degree. C./h and the tubes are sintered at about 1200.degree. C. for 5 to 10 h. All the heatings are done in stagnant air. Performance characteristics of the membranes depend on the stoichiometry of the cations in the ceramic.
In commonly assigned U.S. Pat. No. 5,573,737 to Uthamalingam Balachandran, Joseph T. Dundek, Mark S. Kleefisch and Thaddeus P. Kobylinski, a functionally gradient material is described as including an outer tube of perovskite, which contacts air, an inner tube of zirconium oxide which contacts methane gas, and a bonding layer between the perovskite and zirconium oxide layers.
Even though the functionally gradient oxide materials disclosed in U.S. Pat. No. 5,573,737 exhibit greater stability than other known compositions, there are, under some conditions, certain problems associated with them in the form of unsupported reactor tubes. The reactor tubes can fracture at regions slightly away from the hot reaction zone where temperatures of the tube can, e.g., drop from about 800.degree. C. to about 700.degree. C. in the failure regions.
Accordingly, it is an object of the present invention to provide stable composite materials for membrane reactors which include a gas-tight ceramic having a composition which exhibits both ionic and electronic conductivity as well as appreciable oxygen permeability.
It is another object of the present invention to provide stable composite materials for membrane reactors useful in converting low hydrocarbons to high value products which exhibit greater stability when exposed to a reducing gas environment and other operating conditions for extended time periods.
It is an object of the invention to overcome one or more of the problems described above.
Other objects and advantages of the invention will be apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the drawing and the appended claims.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.